Sound recording



April 27', 1937. J, MAYER 2,078,918

souma RECORDING Filed Feb 11, 1957 babcba'bcb ZSnvehtor Patented Apr. 27, 1937 UNITED sr SOUND RECORDING Application February 11, 1937, Serial No. 125,195

In Germany ()ctober 24, 1935 2 Claims.

This invention relates to the production of sound records, such as those utilized in conjunction with motion pictures, and has for its principal object the provision of an improved and relatively inexpensive sound recording apparatus and method of operation whereby a variable density record of either the class-A or class-B type may be produced.

As used in the following description and claims, the term class-A is intended to designate a sound record including two similar sound tracks displaced in phase by and the term class-B is intended to designate a sound record including two sound tracks, on one of which is recorded the positive half cycles of the sound wave and on the other of which is recorded the negative half cycle of the sound Wave. The class-A and class-B types of records are both classed as push-pull records.

As disclosed by U. S. Patent 1,833,373, a class-A variable density type of push-pull record may be produced by means of two glow lamps which have their illumination modulated by similar sound waves displaced in phase from one another by 180. It has also been proposed to produce a class-A variable area type of push-pull record by means of a sound wave controlled galvoncmeter arranged to vibrate a beam of light lengthwise of a light slit so that the edges of the beam never pass beyond the ends of the slit. The present invention is distinguished from these two different arrangements in that a light beam of constant intensity is' applied to the reflector of a sound wave or audio current controlled galva-nometer either (1) through light wedge elements which have their transparency gradually varied in opposite directions or (2) through light wedge elements which have their transparency gradually varied for one half cycle of the recorded sound Wave and have a constant transparency for the opposite half cycle of this wave. As will appear, the use of the first type of light wedge results in a class-A type of push-pull record, the use of the second type of wedge results in a class-B type of push-pull record, and these two types of light wedges are arranged to be interchanged for producing the difierent types of push pull records.

The invention will be better understood from the following description when considered in connection with the accompanying drawing and its scope is indicated by the appended claims.

Referring to the drawing Fig. 1 illustrates a sound recorder embodying the invention,

Fig. 2 illustrates a type of light Wedge suitable for producing a class-A type of push-pull record.

Fig. 3 illustrates a type of light wedge suitable for producing a class-B type of push-pull record.

Fig. 4 is an explanatory diagram relating to the negative record produced by the light wedge of Fig. 3, and

Fig. 5 is a similar diagram relating to a positive record made from such a negative.

Fig. 1 shows a constant light source I, a condenser lens 2 which concentrates the light of the source I, and projects it through the two light openings l and 5 by means of lens 6 on mirror I. This mirror is movable around an axis, crossing the running direction of the film vertically, and is controlled by the oscillations to be recorded. In diaphragm 3 there are inserted two light absorbing media in openings and 5, e. g. grey wedges. These wedges are mounted in such a way that one has an increasing transparence from the top to the bottom, the other an increasing transparence from the bottom to the top. By means of lens 9 mirror I throws two images t and 5' of the two grey wedges that are in openings i and 5 on slot diaphragm W. The position of the two images l and 5' in relation to slot l l is of such a nature that one half length each of the slot is supplied with light by one grey wedge image, and that at the position of rest of the mirror the mean transparence lies above the slot opening. The upper and lower edge of the images lie parallel to the slot length. With a rotation of mirror 7 the two grey wedge images travel over slot l l viz vertically to the slot length, and according to their position and transparence, more or less light is allowed to pass through the slot. Slot H is portrayed in a reduced size on film I 4 by means of lens 52 at l3; it records two 4 rows of sound tracks with an opposed blackening change, viz a class-A push-pull sound record. on the film. Prism I5 is included merely for reasons of drawing technique for the purpose of illustrating better the ray course, and images l and 5.

In Fig. 2, slot diaphragm It] with slot opening I l, and images l and 5' of the grey wedges is illustrated. The fading transparence of the two grey wedges is indicated in their images i and 5 by the more or less closed hatched design. The height of the two grey wedge images must, of course, be a multiple of that of the slot. In order to obtain the largest possible egress control of the light with the smallest possible motions of the mirror, the grey wedges will be designed with Cit a steeply increasing transparence, and arrangements for their equivalent portraying will be made.

The advantages of the arrangement according 5 to the invention are that only one light source,

and only one controlling or light modulating element are required for recording in accordance with the variable density process. Furthermore, the controlling energy required can be kept at a 10 very small amount because the amplitudes of motion of the mirror can be maintained at a very small figure if correspondingly steep grey wedges are used. Also, the light egress control is greater than with the previous processes be- ]5 cause it exclusively depends on the dimensioning of the blackening range of the grey wedges.

The arrangement can also: be employed for producing a class-B push-pull type sound record with which only the positive half-waves are re- 20 corded on one sound track, only the negative half-waves of the oscillation are recorded on the other sound track, and the blackening or transparency remains constant between the individual half-waves of each sound series. It is preferable that the constant shading between the half cycles on the negative be relatively small or close to the limit of halation, and relatively large, about from 1.0 to 1.2, on the positive. This affords the advantage that as a consequence of the intensive blackening of the positive the ground noise caused by the granulous nature of the photographic layer can be reduced almost to the theoretically possible minimum amount, whereby a purer sound reproduction, and a greater egress control pos- 39 sibility, viz a pure sound effect, is produced with necessity of having the additional control of the light controlling organ otherwise employed in recording. The reason of the greater egress control is that the blackening range of a half-wave 40 is no longer given by the quotient of a maximum blackening in relation to a blackening of rest or of a blackening of rest in relation to a minimum blackening, but by the quotient of a maximum blackening in relation to a minimum 45 blackening.

This half-wave recording can be eilected by the fact that the grey wedges inserted into diaphragm openings 4 and 5 in Fig. 1 are made in such a way that one half has a small transpar- 50 ence, which is constant, according to the halfwave which is not to be recorded, while the other half shows a changeable transparence according to the half-wave to be recorded. In this case, the change from the constant transparence to the 55 changeable transparence does not follow a desultory course, but a fading course, i. e. the course of transparence oi the grey wedge is of such a nature that it first drops to a minimum, and remains constant and equal to the minimum value from that point. The optical image of such grey wedges on the slot diaphragm is shown in Fig. 3, viz again for the position of rest of the mirror. i8 is the slot diaphragm, II the slot opening,and 4 and 5 are the images of the new grey wedges. The portion between a. and b or between a and b" corresponds to the changeable decreasing transparence of the grey wedge, that between I) and c or between I) and c to transparence, remaining equal, of the grey wedge. When portion be or bc travels over slot ll only a small and constant shading is recorded on the negative; in case portion ab or ab travels over the slot, a changeable shading corresponding to a halfwave of the oscillation to be recorded is recorded.

In Fig. 4, the dependence of shading S on the position of the grey wedge images in relation to the slot is represented for the two sound tracks, viz for the negative. Fig. 5 shows the corresponding blackening curves of the positive.

The reproduction of this half-wave record is effected in the same way as that of a push-pull sound record. The two sound tracks are illumined by a light slot, and the light changes of each sound track are carried to one photo-electric cell each. The sound Wave halves displaced by 180 in their phase are again united by an electric push-pull connection to the full oscillation, and the latter is carried to the control grid of the first amplifier tube in a well known manner.

The advantages of this novel process are that it acts as a pure sound process by itself without necessity of having the additional control of the light controlling organ otherwise required in recording. Another advantage of the process is that the shading range which is available for recording a half-wave is doubled, as compared with the previous intensity processes.

I claim as my invention:

1. The combination of means for producing a light beam, means defining a light slit, means for controlling said light beam to produce at said slit images which vary inversely in light intensity transversely of said slit, and means for vibrating said images transversely of said slit.

2. The combination of a source of light, means defining a light slit, means for causing said light to appear at said slit as a pair of images which vary inversely in light intensity transversely of said slit and have opposed constant and variable light intensities longitudinally of said slit, and means for vibrating said images transversely of said slit.

JOSEF MAYER. 

